1. Field of the Invention
The invention relates in general to a signal-transforming device for transforming computer graphics signals to television signals, and more particularly to a signal-transforming device, which can reduce the needed buffers in the process of transforming computer graphics signals to television video signals.
2. Description of the Related Art
In the present post-PC eras, Information Appliance (IA) plays a more and more important role. Many activities, which are generally enjoyed on personal computers (PC) in the past, such as Internet surfing, can currently be enjoyed on some specified-purpose devices, such as set-top box etc. The television is the most popular device for image output in the family. Consequently, in order to make miscellaneous information appliances prevalent in every family, the images originally displayed on the PC should also be able to be displayed on the television. That is to say, the computer graphics signals should be transformed, making it consistent with the standards of the television video signals, and then can be displayed on the television.
The television video signals differ from the computer graphics signals, such as dimensions and interlacing or not. There are different standards for television video signals, such as NTSC, PAL, and so on. Take NTSC for example, there are totally 525 vertical lines, and approximately 420 ones among them are in the displayed area, depending on the type of the television. However, the resolution of the PC is 640×480, 800×600 and so on. Therefore, the computer graphics signal should be scaled down first and then can be displayed on the television.
On the other hand, the way of television display adopts the method of interlace scanning. That is to say, every frame of the television video signals is composed of one even field and on odd field. In the even field, every other TV line is scanned, as shown in FIG. 1A; in the odd field, the TV line between the two TV lines in the even field is scanned, as shown in FIG. 1B. After the scanning of the even field is completed, the scanning of the odd field is then proceeded, completing one full frame. However, the way of computer display adopts the method of Non-Interlace scanning. In every frame, every computer vertical line is scanned one by one, shown as FIG. 1C.
Therefore, in order to be displayed on the television, the computer graphics signals must go through two-step signal processing. The first step is to scale down the computer graphics signals. Take the 640×480 computer graphics signals and the NTSC television for example, the number of the computer vertical lines is 480 and that of the TV lines is 420, with the ratio 8:7. That is to say, in order to conform to the standards of the TV, every 8 vertical lines of computer graphics signals should be scaled down to the corresponding 7 TV lines of television video signals. In order to improve the video quality, conventionally the weighted-averages method is used for down scaling. Referring to FIG. 2, which shows the block diagram of the conventional scaling down operation for computer graphics signals. Every scaled-down vertical line L′(n) is obtained from two vertical lines of computer graphics signals by using the weighted averages method, i.e.L′(n)=a(n)L(n)+b(n)L(n+1), wherein a(n)+b(n)=1.  (1)
As shown in FIG. 2, L′(0)=L(0)+0*L(1), L′(1)=(6/7)L(1)+(1/7)L(2), L′(2)=(5/7)L(0)+(2/7)L(1), and the rest may be deduced by analogy.
Then, what follows is the second step operation, that is, interlacing the scaled-down computer graphics signals to individually produce the TV lines of the even and odd field. By doing so, the transformation of computer graphics signals to television video signals is completed, as shown in FIG. 3 Conventionally one TV line is obtained from three adjacent scaled-down vertical lines of computer graphics signals by using the weighted-averages method. The relationship between the TV line L″(n) and the scaled-down vertical line L′(n), L′(n+1), L′(n+2) is shown in the following equation.L″(n)=αL′(n)+βL′(n+1)+γL′(n+2), wherein α+β+γ=1  (2)
Referring to FIG. 4, which shows the block diagram of the conventional circuits for transforming computer graphics signals to television video signals. The initial buffers 402 and 404 store the data of the vertical lines of computer graphics signals. The scaled-down buffers 406, 408, and 410 store the data of the scaled-down vertical lines of computer graphics signals. The interlacing buffer 412 stores the data of the TV lines, which are obtained from interlacing the scaled-down vertical lines of computer graphics signals. After the two vertical lines stored in the initial buffers 402 and 404 go through the operation of the weighted-averages unit 414, the scaled-down vertical line is generated, which is then stored in the scaled-down buffers 406, 408, or 410 through the operation of the controller 416. Then, the scaled-down vertical lines stored in the scaled-down buffers 406, 408 and 410 go through the operation of the weighted-averages unit 418, and then one TV line is obtained. Then, the TV lines are stored in the interlacing buffer 412 and output as the TV lines of the odd or even field of the TV frame. (In general, all the TV lines of the even field are first obtained from all the vertical lines of one computer frame, and then all the TV lines of the odd field are obtained from all the vertical lines of the same computer frame.) Therefore, when the foregoing operations are performed in proper sequence to all vertical lines of one computer frame, the transformation of one frame of computer graphics signals to television video signals is completed.
Six buffers are totally needed in the conventional circuits for transforming computer graphics signals to television video signals. Take the computer vertical line, which includes 640 pixels, for example. If every pixel uses 1 byte memory space, then every buffer needs at least 640 bytes memory space. That is, six buffers need at least 3840 bytes memory space. In integrated circuit (IC) design, for the whole IC, the proportion of space and cost occupied by the memory is very high. Therefore, in the conventional method, the memory space needed for the six buffers is very large, and likewise the cost is very high. Therefore, it is necessary to find other methods to reduce the needed buffers in the transformation process.